1. Field of the Invention
The present invention relates to a reversible thermosensitive recording medium comprising a reversible thermosensitive coloring composition which comprises an electron-donating coloring compound and an electron-accepting compound, and is capable of inducing color formation by utilizing the coloring reaction between the electron-donating compound and the electron-accepting compound. The recording medium is therefore capable of repeatedly forming a colored image therein and erasing the same therefrom by controlling the thermal energy applied thereto.
2. Discussion of Background
There is conventionally known a thermosensitive recording medium utilizing the coloring reaction between an electron-donating coloring compound (hereinafter referred to as a coloring agent or leuco dye) and an electron-accepting compound (hereinafter referred to as a color developer). This kind of thermosensitive recording medium is widely used, for example, in a facsimile machine, a word processor, and a printer of a scientific measuring instrument. However, the coloring reaction of the conventional thermosensitive recording medium of this type has no reversibility, so that color development and decolorization cannot be alternately repeated.
Among published patents, a recording medium capable of reversibly carrying out the color development and decolorization is also proposed. For example, a thermosensitive recording medium using phloroglucinol and gallic acid in combination as color developers is disclosed in Japanese Laid-Open Patent Application 60-193691. There is disclosed in Japanese Laid-Open Patent Application 61-237684 a reversible thermosensitive recording medium which employs compounds such as phenolphthalein and thymolphthalein as color developers. In Japanese Laid-Open Patent Applications 62-138556, 62-138568, and 62-140881, there are disclosed reversible thermosensitive recording media, each comprising a recording layer which contains a homogeneously dissolved composition of a coloring agent, a color developer, and a carboxylic acid ester. Further, in Japanese Laid-Open Patent Application 63-173684, a reversible thermosensitive recording medium comprising as a color developer an ascorbic acid derivative is disclosed. In addition, a reversible thermosensitive recording medium comprising as a color developer a salt of bis(hydroxy-phenyl)acetic acid or gallic acid, and a higher aliphatic amine is disclosed in Japanese Laid-Open Patent Applications 2-188293 and 2-188294.
Further, the inventors of the present invention proposed a reversible thermosensitive coloring composition which comprises an organic phosphoric acid compound, an aliphatic carboxylic acid compound, or a phenol compound, each having a long-chain aliphatic hydrocarbon group therein, as a color developer, and a leuco dye as a coloring agent, as disclosed in Japanese Laid-Open Patent Application 5-124360. By use of such a reversible thermosensitive coloring composition, the color development and the decolorization can be easily carried out by controlling the heating and cooling conditions for the coloring composition, and further, the color-developed state and the decolorized state can be stably maintained at room temperature, and the color development and the decolorization can be alternately repeated in a stable condition. In this application, there is also proposed a reversible thermosensitive recording medium which comprises a recording layer containing the above-mentioned reversible thermosensitive coloring composition. The present invention is based on the invention disclosed in Japanese Laid-Open Patent Application 5-124360, which has been issued as Japanese Patent No. 2981558, U.S. Pat. Nos. 5,395,433, 5,432,534, and 5,296,439, DP No. 69120645, and FP E 0492628.
Further, the use of a phenol compound having a long-chain aliphatic hydrocarbon group with a specific structure as the color developer is proposed in Japanese Laid-Open Patent Application 6-210954.
However, the above-mentioned conventional reversible thermosensitive recording media have the drawbacks that the image density is decreased and the printed image is deformed as image formation and erasure is repeated under the practical conditions. Namely, the color development and decolorization characteristics peculiar to each coloring composition cannot be constantly exhibited during the repeated operations. This is because not only the thermal energy, but also a mechanical stress is applied to the recording medium when thermal printing is carried out using a thermal head. The result is that the structures of the layers constituting the recording medium, such as a recording layer and a protective layer, are unfavorably damaged during the repeated operations.
To solve the above-mentioned problem, Japanese Laid-Open Patent Application 6-340171 aims to improve the repeated use durability of the recording medium by adding particles to the recording layer, the particles having an average particle size that is 1.1 times the thickness of the recording layer. Japanese Laid-Open Patent Application 8-156410 discloses a reversible thermosensitive recording medium which comprises a protective layer with a specific glossiness and surface roughness in order to improve the head-matching properties, and repeated use durability.
Even though the above-mentioned recording layer or protective layer is employed, the damage of these layers cannot be completely prevented during the repeated image formation and erasure operations. As a result, the surface of the recording medium is bruised by the application of thermal energy and mechanical stress in the repeated use, so that the printing quality is impaired. Therefore, the permissible number of repeated operations of image formation and erasure is limited in the practical use.
Furthermore, the recording medium is usually irradiated with light while operated and stored. In view of this, the recording medium is required to have high light resistance to such a degree that the color development and decolorization characteristics do not deteriorate by exposure to light. However, in fact, when the recording medium is exposed to a fluorescent lamp or sunlight for a long time, color change occurs at a colored image portion and a background portion. In particular, there is a tendency that the colored image is not thoroughly erased, and faintly remains after image erasure operation.
Japanese Laid-Open Patent Applications 8-310128, 9-272262, 9-270563, 9-300817, and 9-300820 propose the addition of a specific decolorization accelerating agent to the recording layer in order to obtain instantaneous decolorization properties. However, there are the problems that a colored image is not perfectly erased when an electron-accepting compound for causing the reversible color change and a binder agent are replaced. Further, no improvement is observed in the characteristics of the decolorization initiating temperature and the decolorization temperature range.
Accordingly, a first object of the present invention is to provide a reversible thermosensitive recording medium with instantaneous decolorization properties, excellent color development properties, and preservation stability, and repeated use durability without causing no bruise on the surface of the recording medium.
A second object of the present invention is to provide an image display apparatus using the above-mentioned reversible thermosensitive recording medium.
A third object of the present invention is to provide an image display method using the above-mentioned reversible thermosensitive recording medium as a rewritable paper.
The above-mentioned first object of the present invention can be achieved by a reversible thermosensitive recording medium comprising a support, and a reversible thermosensitive recording layer formed thereon comprising:
a reversible thermosensitive coloring composition comprising an electron-donating coloring compound and an electron-accepting compound, which composition is capable of inducing color formation in the electron-donating coloring compound, capable of reversibly assuming a color-developed state or a decolorized state, depending upon the temperature of the reversible thermosensitive coloring composition when heated, and/or the cooling rate of the reversible thermosensitive coloring composition when cooled after the heating thereof, and
a decolorization accelerating agent represented by formula (I): 
wherein p is an integer of 0 to 3; q and r are each independently an integer of 1 to 3; X1, X2, and X3 are each a bivalent hetero-atom-containing group; R1, R2, and R3 are each a hydrocarbon group having 1 to 22 carbon atoms, provided that at least one of R1, R2, or R3 is an aliphatic hydrocarbon group having 8 or more carbon atoms; and R4 is a hydrogen atom, hydroxyl group, or an aliphatic hydrocarbon group having 1 to 8 carbon atoms.
The above-mentioned reversible thermosensitive recording medium is found to have quick response to the change of applied thermal energy, thereby achieving instantaneous decolorization. Further, the preservation stability, in particular, thermal stability of the recording medium is remarkably improved.
The second object of the present invention can be achieved by an image display apparatus for displaying an image, using as an image display medium the above-mentioned reversible thermosensitive recording medium.
The third object of the present invention can be achieved by an image display method for displaying an image on the above-mentioned reversible thermosensitive recording medium, comprising the step of forming an image in the above-mentioned recording medium corresponding to the output information in such a manner that heat is imagewise applied to the recording medium.
The above-mentioned image display method may further comprise the step of erasing the displayed image in such a manner that heat is applied to the recording medium.
According to the image display method, information can be repeatedly recorded in the reversible thermosensitive recording medium and erased therefrom, so that the reversible thermosensitive recording medium can be used as a rewritable paper.